Gas turbines are widely used to provide standby or supplemental peak power for electric utilities. Gas turbines are preferred for these uses because of their rapid starting capability and low capital cost. The thermal efficiency of a conventional gas turbine by itself, however, is typically low due to the high temperature of the exhaust gas. The gas turbine can be combined with a heat recovery steam generator to recover exhaust heat and therefore improve the overall thermal efficiency of the combined cycle system. The heat recovery steam generator can be employed to run a steam turbine or to provide process steam. The combination of the gas turbine and the heat recovery steam generator results in a system referred to typically as a combined cycle system.
The heat recovery steam generator is positioned downstream in the exhaust gas direction from the gas turbine. In one form of heat recovery steam generator, the housing defines a generally horizontal exhaust gas flow path. The cross-sectional area of the gas turbine exhaust is typically small in comparison to the full cross-section of the main body of the heat recovery steam generator. Therefore, the housing of the heat recovery steam generator typically has an inlet transition or diffuser portion connecting the duct, at the outlet of the gas turbine, to the main portion or full cross-section portion of the heat recovery steam generator. Heat recovery assemblies, typically including reheaters, superheaters, evaporators, economizers and feedwater heaters, are positioned in the full cross-section portion of the housing for the recovery of thermal energy from the exhaust gas stream. Furthermore, pollution control assemblies are positioned in the full cross-section portion of the housing for exhaust emission reduction. A supplementary firing system, typically a duct burner, can be located in the diffuser portion of the housing to consume excess air in the exhaust gas stream and therefore increase steam output from the heat recovery steam generator.
The expansion angle of the diffuser portion is typically 30.degree. or greater due to space and cost constraints. As a result of the rapid expansion of the exhaust gas stream in the diffuser portion of the housing, the exhaust gas stream can undergo flow separation. This flow separation results in a non-uniform flow distribution of the exhaust gas stream entering the full cross-section portion of the heat recovery steam generator. The non-uniform flow distribution degrades performance of the duct burners and the pollution control devices of the heat recovery steam generator. Furthermore, the efficiency of heat recovery in the heat recovery steam generator itself can be reduced to unacceptable levels due to poor flow distribution of the exhaust gas stream. In addition, non-uniform flow distribution of the exhaust gas stream in the diffuser portion of the housing can increase the system pressure drop and therefore reduce the overall efficiency of the combined cycle system.